The present invention relates to an active matrix type liquid crystal display apparatus for use in a liquid crystal television set, a notebook personal computer, and the like.
FIGS. 24 and 25 are a plan view and a sectional view, respectively, of a conventional active matrix type liquid crystal display apparatus. The active matrix type liquid crystal display apparatus is constituted essentially of a liquid crystal panel 1, a gate driver 2, a source driver 3, and a backlight 4.
The liquid crystal panel 1 has an active matrix board 5, an opposed board 6, a liquid crystal layer 7 sandwiched between the active matrix board 5 and the opposed board 6, and a polarizer (not shown) attached to the outer side of each of the active matrix board 5 and the opposed board 6.
On an insulation substrate 5a of the active matrix board 5, there are provided a plurality of scanning lines (not shown) disposed parallel with one another, a plurality of signal lines 9 parallel with one another and orthogonal to the scanning lines with an insulation film 8 disposed between the signal lines and the scanning lines, thin film transistors (TFTs) 10 disposed in the vicinity of intersections of the scanning lines and the signal lines 9, and a plurality of pixel electrodes 11 disposed in regions surrounded with the scanning lines and the signal lines 9.
FIG. 26 is a plan view showing a one-pixel part of the active matrix board 5. Because the pixel electrode 11 and the signal line 9 are formed in the same layer, the pixel electrode 11 is spaced at a predetermined interval from the signal line 9 to prevent the pixel electrode 11 from contacting the signal line 9. In the TFT 10 which is a three-terminal element, electrical continuity between a drain electrode 13 and a source electrode 14 is controlled by a voltage applied to a gate electrode 12. The gate electrode 12 is connected to a scanning line 15 adjacent thereto. The source electrode 14 is connected to the signal line 9 adjacent thereto. The drain electrode 13 is connected to the pixel electrode 11.
The opposed board 6 is provided with color filters 16 formed in the order of red, green, and blue at positions corresponding to each pixel electrode 11. A black matrix 17 is formed between the adjacent color filters 16 and 16. The black matrix 17 serves as a light shield film for preventing leak of light from the gap between the pixel electrode 11 and the scanning line 15 as well as the signal line 9. An opposed electrode 18 made of a transparent conductive material is formed on a layer of the black matrix 17 and the color filters 16. The gate driver 2 and the source driver 3 are connected to terminals of the scanning lines 15 and those of the signal lines 9, respectively, disposed on the periphery of the liquid crystal panel 1.
The method of driving the active matrix type liquid crystal display apparatus having the construction will be described below.
When writing to an array of pixels of an nth row, an ON-signal (electric potential Vgh at which the TFT 10 is turned on) is input to a scanning line 15n of the nth row from the gate driver 2. At this time, an OFF-signal (electric potential Vg1 at which the TFT 10 is turned off) is input to scanning lines other than the scanning line 15n. Thus, only the TFTs 10 of the nth row are turned on. On the other hand, source signals having voltages to be applied to the pixels (pixel electrodes 11 and liquid crystal layer 7) of the nth row are supplied to each signal line 9 from the source driver 3.
Upon completion of write for the array of the pixels of the nth row terminates, the OFF-signal is input to the scanning line 15n, whereas the ON-signal is input to the next scanning line 15(n+1). All pixels are charged with voltages corresponding to data by repeating the operation. The transmissivity of the liquid crystal layer 7 disposed between the pixel electrode 11 and the opposed electrode 18 changes depending to a voltage applied across the pixel electrode 11 and the opposed electrode 18, and light emitted from the backlight 4 is therefore adjusted. As a result, images are displayed on the active matrix type liquid crystal display apparatus.
There is proposed a construction in which pixel electrodes are provided on an interlaminar insulation film so that the pixel electrodes and the signal line are formed as different layers and that the pixel electrodes overlap the signal lines (disclosed in Japanese Patent Application Laid-Open No. 63-279228). FIG. 27 is a sectional view showing a one-pixel part of an active matrix type liquid crystal display apparatus having the above-mentioned construction in which pixel electrodes overlap signal lines. FIG. 28 is a plan view of an active matrix board 24 shown in FIG. 27. In the construction, pixel electrodes 21 and signal lines 22 are formed as separate layers, and the pixel electrodes 21 are overlaid on the signal lines 22 through an interlaminar insulation film 23. Thus, it is possible to eliminate the gaps between the pixel electrodes 21 and the adjacent signal lines 22. Thus, it is possible to enlarge the area of the pixel electrodes 21 (aperture ratio) and thus reduce the power consumption of the active matrix type liquid crystal display apparatus. In FIGS. 27 and 28, reference numeral 24a denotes an insulation substrate, 25 denotes a TFT, 26 denotes a liquid crystal layer, 27 denotes an opposed electrode, 28 denotes an opposed board, 29 denotes a scanning line, 30 denotes a contact hole, 31 denotes an auxiliary capacitor electrode, and 32 denotes an auxiliary capacitor line.
However, in comparison with the construction shown in FIG. 26 in which the pixel electrode 11 is spaced at a predetermined interval from the signal line 9, the construction in which the pixel electrodes 21 overlap the signal lines 22 invites an increased capacitance Csd between the pixel electrode 21 and the signal line 22. With the increase of the capacitance Csd, the source signal causes a pixel electric potential to change easily. Eventually, there will occur display characteristic deterioration called shadowing phenomenon.
The mechanism of the shadowing phenomenon will be described below by using an equivalent circuit of the active matrix board 24 shown in FIG. 29. When a TFT 25 is turned on as a result of input of an ON-signal Vgh to a scanning line Gn, a pixel electrode P1 is supplied with a voltage Vs1 from a signal line S1.
Next, when the TFT 25 is turned off as a result of input of an OFF-signal Vg1 to the scanning line Gn, a voltage Vs1′ corresponding to data to be written to a pixel electrode P2 of a next stage is supplied to the signal line S1. At this time, the voltage of the pixel electrode P1 is influenced by the voltage Vs1′ of the signal line S1 through the capacitance Csd1. Supposing that the voltage of the pixel electrode P1 at that time is Vp1, the voltage Vp1 is expressed as follows:Vp11=Vs1−(Csd1(Vs1−Vs1′)+Csd2(Vs2−Vs2′))/(Cp+Csd1+Csd2)  (1)where Cp is a capacitance of the pixel electrode (Cp=liquid crystal capacitance, Clc+auxiliary electrode capacitance, Ccs), Csd1 is a capacitance between the signal line S1 and the pixel electrode P1, Csd2 is a capacitance between a signal line S2 and the pixel electrode P1, Vs1 and Vs2 are voltages of the signal lines S1 and S2, respectively, in the case where the scanning line Gn of an nth row is in an ON-state, and Vs1′ and Vs2′ are voltages of the signal lines S1 and S2, respectively, in the case where a scanning line G(n+1) of an (n+1)th row is in an ON-state.
In a gate line inversion driving method (namely, “1H inversion driving”) which is a conventional method of driving the active matrix type liquid crystal display apparatus, the polarity of the source signal is inverted every line of gates. Supposing that adjacent gradations are the same,Vs=Vs1=Vs2, Vs′=Vs1′=Vs2′  (2)Therefore, from the equations (1) and (2),Vp1=Vs−(Csd1+Csd2)/(Cp+Csd1+Csd2)·(Vs−Vs′)  (3)
As is obvious from the above, in the 1H inversion driving, the amount of change of the pixel electric potential is proportional to (Csd1+Csd2). Therefore, with the increase of the capacitance Csd between the signal line S and the pixel electrode P, the shadowing phenomenon appears conspicuously.
A dot inversion driving method has been proposed as a driving method suppressing the change of the pixel electric potential due to the capacitance Csd between the signal line S and the pixel electrode P. In the dot inversion driving, the polarity of the source signal is inverted not only every line of the gates, but also every line of sources.
Supposing that adjacent gradations are the same in the dot inversion driving,Vs=Vs1=−Vs2, Vs′=Vs1′=−Vs2′  (4)From the equations (1) and (4),Vp1=Vs−(Csd1−Csd2)/(Cp+Csd1+Csd2)·(Vs−Vs′)  (5)
From the above, in the dot inversion driving, the variation of the pixel electric potential is proportional to the difference between the capacitance Csd1 and the capacitance Csd2. Therefore, the dot inversion driving is much superior to the 1H inversion drive in suppressing the occurrence of the shadowing phenomenon. Thus, the dot inversion driving can improve the image quality of the liquid crystal display apparatus. In particular, by reducing the difference between the capacitances Csd1 and Csd2 in connection to the pixels adjoining in the direction in which the scanning line 29 extends, it is possible to suppress the occurrence of the shadowing phenomenon to a great extent.
However, the following new problem occurs. In general, in producing a liquid crystal display apparatuses, a photolithographic process is performed block by block. Thus, an alignment deviation occurs from block to block. This leads to the variation in the amount of overlapping between the pixel electrode P and the signal line S and hence the variation in the capacitance Csd between the signal line S and the pixel electrode P. In the case where the dot inversion driving is adopted, the pixel electric potential is liable to change due to the variation in the capacitance Csd. This results in difference of transmissivity among the blocks.
For example, referring to FIG. 30, let it be supposed that an alignment deviation dx has occurred in the photolithographic process of the pixel electrodes P. In this case, there is an increase in the amount of overlapping between the pixel electrode P and the signal line S1. Thus, there is an increase in the capacitance Csd1 between the signal line S1 and the pixel electrode P, whereas there is a decrease in the capacitance Csd2 between the signal line S2 and the pixel electrode P. FIG. 31 shows the relationship between the alignment deviation dx in the photolithographic process and the capacitances Csd1, Csd2. FIG. 31 indicates that with the increase of the alignment deviation dx, the difference between the capacitance Csd1 and the capacitance Csd2 becomes big, and the amount of variation of the pixel electric potential increases.
In the ordinary conventional photolithographic process, the surface of the active matrix board is exposed in blocks. This is the reason why, if a deviation dx occurs in the alignment, the amount of overlap of the pixel electrode on the signal line differs from block to block and the transmissivity differs among the blocks of the active matrix type liquid crystal display apparatus. FIG. 32 shows the relationship between the alignment deviation dx and the difference ΔT in transmissivity between a block having the alignment deviation dx and a block having no alignment deviation.
As is obvious, if the active matrix type liquid crystal display apparatus in which the pixel electrodes overlap the signal lines is driven by the dot inversion driving method, the amount of change in pixel electric potential caused by the coupling capacitances Csd really decreases, but differs largely among the photo-blocks. Consequently, there rises a big difference in the transmissivity among the blocks, leading to a problem called “block separation”. As the size of the active matrix type liquid crystal display apparatus becomes larger, the number of blocks tends to increase more and more in the photolithographic process. Thus, there is a growing demand for suppression of the occurrence of the “block separation” caused by the coupling capacitance Csd.